Slurry phase polymerisation of olefins is well known wherein an olefin monomer and optionally olefin comonomer are polymerised in the presence of a catalyst in a diluent in which the solid polymer product is suspended and transported.
The present invention is more particularly concerned with polymerisation in a loop reactor where the slurry is circulated in the reactor typically by means of a pump or agitator. Liquid full loop reactors are particularly well known in the art and are described for example in U.S. Pat. Nos. 3,152,872, 3,242,150 and 4,613,484.
Polymerisation is typically carried out at temperatures in the range 50-125° C. and at pressures in the range 1-100 bara. The catalyst used can be any catalyst typically used for olefin polymerisation such as chromium oxide, Ziegler-Natta or metallocene-type catalysts. The product slurry, comprising polymer and diluent and in most cases also catalyst, olefin monomer and comonomer can be discharged intermittently or continuously, optionally using concentrating devices such as hydrocyclones or settling legs to minimise the quantity of fluids withdrawn with the polymer.
The loop reactor is of a continuous tubular construction comprising at least two, for example four, vertical sections and at least two, for example four, horizontal sections. The heat of polymerisation is typically removed using indirect exchange with a cooling medium, preferably water, in jackets surrounding at least part of the tubular loop reactor. The volume of the loop reactor can vary but is typically in the range 20 to 250 m3; the loop reactors of the present invention are of this generic type.
Maximum commercial scale plant capacities have increased steadily over the years. Growing operating experience over the last few decades has led to operation of increasingly high slurry and monomer concentrations in reaction loops. The increase in slurry concentrations has typically been achieved with increased circulation velocities achieved for example by higher reactor circulation pump head or multiple circulation pumps as illustrated by EP 432555A and EP 891990A. The increase in solids loading is desirable to increase reactor residence time for a fixed reactor volume and also to reduce downstream diluent treatment and recycling requirements (a higher solids concentration obviously corresponds to a reduced proportion of diluent). The increased velocity and pressure drop requirement of the loop has however led to increasing pump design sizes and complexity, and also increasing energy consumption as slurry concentrations increase. This has both capital and operating cost implications.
Historically the circulation velocity in the reaction loop has typically been maximised to ensure maintenance of good thermal, compositional and particle distribution across the reactor cross-section, particularly the avoidance of solids settling, stable flow characteristics, or excessive solids concentrations at the pipe wall. Inadequate cross-sectional distribution could lead to increased fouling, reduced heat transfer and reduced polymer productivity and homogeneity. For example, WO 2004024780 discloses in Tables 2 and 3 circulation velocities of at least 6.9 m/s in order to avoid saltation, which is the phenomenon of particles bouncing along the wall of the reactors rather than being wholly suspended in the diluent.
We have found that it is in fact possible to operate at lower circulation velocities and/or higher solids concentrations than typically used in the prior art by careful optimisation of the geometry of the reactor, so as to minimise the degree of uneven cross-sectional distribution of solid polymer. More specifically, we have found that it is possible to calculate the effect of gravitational and centrifugal forces acting on the slurry which contribute to an uneven cross-sectional distribution of solids, so that the reactor geometry can be designed such that the net effect of those forces in any particular direction is minimised.
The present invention therefore provides a process for polymerising, in a loop reactor, at least one olefin monomer in a liquid diluent to produce a slurry comprising solid particulate olefin polymer and said diluent, wherein the ratio between the actual volumetric solids concentration of the slurry and the maximum possible geometric volume solids concentration of the slurry as measured by the bulk density of an unpacked settled bed of particles, SVCR, is V*0.065 or greater, and the ratio of the cumulative settling distance of an average size particle at any point in the reactor in any direction perpendicular to the direction of the flow, to the diameter of the loop reactor, is maintained below [0.084*(V−6.62)+(0.69−SVCR)*1.666], where V is the circulation velocity of the slurry in m/s and “cumulative settling distance” is defined as the cumulative distance, expressed as a fraction of the diameter, travelled by a particle in any direction perpendicular to the direction of the flow since the previous upstream pump. It is preferred that V is less than 9.5 m/s. It is also preferred that the ratio of the cumulative settling distance of an average size particle at any point in the reactor in any direction perpendicular to the direction of the flow, to the diameter of the loop reactor, is maintained below 0.37.